1. Field of the Invention
The present invention concerns a multi-layer wiring board. More specifically, the invention concerns a multi-layer wiring board having a large number of layers capable of being easily produced with a high yield, at a low cost and at one time. The invention further concerns a method of easily producing a multi-layer wiring board having a large number of layers with a high yield, at a low cost and at one time.
2. Description of the Related Art
In recent years, electronic parts have been produced in small sizes and in reduced weights, and an increase of the densities of the printed wiring boards are being accelerated.
As is well known, a printed wiring board of a high density includes a build-up wiring board. The build-up wiring board is conventionally produced by forming a lower-layer circuit on the surface of a copper-lined laminated plate, flattening the surface of the laminated plate with a resin, forming an upper-layer circuit on the flattened surface, and electrically connecting the upper-layer circuit to the lower-layer circuit through a via-hole (filled via) filled with a conductor. By repeating a series of these production steps, further, there can be produced a wiring board of a multi-layer structure, i.e., multi-layer wiring board. According to this build-up production method, however, the circuits are successively laminated on a piece of substrate, i.e., the layers are successively laminated on upon another. Therefore, if there occurs a failure during the production of the multi-layer wiring board (according to the prior art method, failure often occurs in forming the circuit), the whole substrate being produced becomes defective. That is, the final yield becomes a product of the yields of each of the steps. As a result, therefore, the yield drops and the cost of the product increases.
To avoid the problems inherent in the above-mentioned prior art method of successively laminating the layers, a method for laminating a plurality of circuit boards at one time has been developed and put into practical use. This method is called a batch lamination method according to which a circuit board is fabricated by forming circuit(s) and filled via(s) for each of the copper-lined laminated plates, a predetermined number of circuit boards are laminated via an adhesive layer and then the laminated product is heated and press-adhered. According to this batch lamination method, however, it is necessary to provide a low-melting point metal layer on an end surface of the filled via to use the metal layer for junction to the circuit board after the circuit board has been formed, and thus there arises the problems of requiring complex production steps and driving up the cost of production. Further, when the adhesive layer is used, the electric connection between the upper circuit board and the lower circuit board is adversely effected by the flow of the adhesive.
Described below are some examples of the prior art batch lamination method involving the above-mentioned problems. Japanese Unexamined Patent Publication (Kokai) No. 7-240582 discloses a multi-layer wiring board obtained by laminating two or more ceramic insulating films each having a wiring formed on either the front surface or the back surface thereof and a filled via. The individual insulating films are coated with an adhesive, laminated, heated and press-adhered together at one time.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2000-13023discloses a method of producing a multi-layer printed wiring board comprising the steps of:
impregnating a nonwoven aramide fabric with a thermosetting resin, forming through-holes therein when it is half-cured, filling the resulting through-holes with an electrically conducting paste to form an adhesive sheet for use as an interlayer insulating sheet;
forming the inner layer members having an electrically conducting pattern on both surfaces of an insulating substrate that has holes filled with the electrically conducting paste;
alternately laminating the inner layer members and the adhesive sheets for insulating the interlayers in a plurality of number of pieces; and
forming through-holes that penetrate through the obtained laminate thereby to form an outer-layer conductor circuit.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2000-269647 discloses a multi-layer printed wiring board produced by successively laminating a plurality of single-sided circuit boards for lamination each comprising an insulating resin base material (e.g., nonwoven base material impregnated with a resin), a conductor circuit formed on one surface thereof and a filled via penetrating therethrough, via an adhesive layer which is applied to a single-sided circuit board for a core, and heat-pressing them together with the once application of pressure.
Further, Japanese Unexamined Patent Publication (Kokai) No. 2001-15913 discloses a multi-layer printed wiring board produced by laminating a plurality of single-sided circuit boards each comprising an insulating base material of a glass epoxy composite material, a conductor circuit formed on one surface thereof and a filled via penetrating therethrough, and heating and pressing them together via an adhesive at one time.
Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2001-160686 discloses a method of producing a multi-layer printed wiring board by laminating, at one time, a first printed board and a second printed board each having a conductor layer on one surface thereof. Each printed board has a via-hole filled with a conductor. The printed boards are bonded together by using an adhesive.
It is an object of the present invention to provide a multi-layer wiring board having a large number of layers capable of easily producing at one time, at a high yield and at a low cost.
Another object of the present invention is to provide a multi-layer wiring board in which the upper and lower circuit boards are electrically connected with a high degree of reliability.
A further object of the present invention is to provide a method of producing a multi-layer wiring board having a large number of layers at a high yield and at a low cost with a batch lamination method.
A still further object of the present invention is to provide a method of producing a multi-layer wiring board in which the upper and lower circuit boards are electrically connected with a high degree of reliability.
The above objects and other objects of the present invention will be easily understood from the following detailed description of the present invention.
The present invention, in one aspect thereof, resides in a multi-layer wiring board which comprises a laminated product of a plurality of insulating layers having conductor circuits, wherein the conductor circuits of the insulating layers are electrically connected together through conductor-filled via-holes of an insulating connection layer having no conductor circuit disposed between the insulating layers, connection surfaces of the conductor circuits and connection surfaces of the conductors in the via-holes are joined together by a metal bonding portion formed by diffusing, in the conductor of the conductor circuit of the insulating layers, a junction conductor which may be the same as, or is different from, the conductor filled in said via-holes of the connection layers, and
the regions other than the conductor circuits of the insulating layers and the regions other than the via-holes of the connection layers are directly joined together by press-adhering the insulating resins that constitute the respective layers.
The present invention, in another aspect thereof, resides in a method of producing a multi-layer wiring board which comprises a laminated product of a plurality of insulating layers having conductor circuits, comprising the steps of:
electrically connecting the conductor circuits of the insulating layers together through conductor filled via-holes of an insulating connection layer having no conductor circuit disposed between the insulating layers, wherein
connection surfaces of the conductor circuits and connection surfaces of the conductors in the via-holes are jointed together by a metal bonding portion formed by diffusing, in the conductor of the conductor circuits of the insulating layers, a junction conductor which may be the same as, or is different from, the conductor filled in the via-holes of the connection layers, and the regions other than the conductor circuits of the insulating layers and the regions other than the via-holes of the connection layers and directed joined together by press-adhering the insulating resins that constitute the respective layers.